1. Field of the Invention
The present invention relates to an optical device and a wavelength division multiplexing communication system using the same.
2. Description of the Related Arts
Recently, with the widespread use of internet and image transmission, the development and introduction of a wavelength division multiplexing (WDM) optical communication are being pushed ahead to meet the explosive growth of communication demand. Especially, as to the WDM system, further larger capacity transmission is being pursued by means of enlarging the number of wavelengths and improving the transmission rate per wave, and additional studies are being made to realize super long distance transmission for somewhere around 2000 km, without the need to insert an expensive regenerative repeater with 3R (receive/regenerate/relay functions).
In transmitting such a super long distance, problems could be the accumulation of channel wavelength-to-wavelength level deviation and the accumulation of channel-to-channel wavelength dispersion deviation.
In addition, channel wavelength-to-wavelength level deviation will be generated by the gain deviation of the optical amplifier for WDM, as well as non-linear effects including the Raman effect scattering (SRS: Stimulated Raman Effect Scattering) which causes amplification effect to the signal on the longer wavelength side, with the optical signal on the shorter wavelength side becoming a pumping light.
For this reason, in the channel signal where the level becomes lower, the optical SNR (signal-to-noise ratio) will be degraded. Due to the degraded ratio, the transmission distance has to be limited.
In the high-speed transmission system, the management of dispersion amount will be additionally important. As the optical receiver has an allowable capacity for wavelength dispersion, the amount of dispersion should necessarily be controlled to 1000 ps/nm maximum, when the speed is 10 Gb/s.
On the other hand, dispersion compensation is totally carried out by inserting a dispersion compensation fiber (DCF) into each in-line amplifier to compensate for the dispersion coefficient of the transmission path fiber which intrinsically exists, depending on the types of transmission path fiber. Moreover, as the dispersion coefficient of the transmission path fiber has a slope to the wavelength, the design is made so that the dispersion deviation at each wavelength is compensated, allowing the dispersion compensation fiber to have an inverted slope.
However, for the NZ-DSF (Non Zero-Dispersion Shift Fiber) currently being used, it is difficult to perfectly compensate up to the dispersion slope with the use of the dispersion compensation fiber. Due to this difficulty, as long distance transmission is being in progress, each wavelength-to-wavelength dispersion deviation will be increased, so the deviation could exceed the allowable dispersion capacity of the optical receiver.
Therefore, to solve the accumulation problem of such level deviation and dispersion slope deviation, studies are being made to place a compensation node at every several spans. FIG. 1 shows a configuration example of an optical multiplexing transmission system with compensation nodes.
In the basic line-based optical communication system, an example of which is shown in FIG. 1, the terminal station A and the terminal station B are connected with a super long distance optical transmission path, about 1,500 km long. Assuming that signals are to be transmitted from the terminal station A to the terminal station B, at the terminal station A, optical signals, each having different wavelength will be inputted by the optical transmitter 100. Then, the optical signals with two or more wavelengths will be wavelength multiplexed with the optical multiplexer 101 and sent out to the transmission path fiber 102.
To the transmission path fiber 102, the optical amplifier 103 will be inserted to maintain the optical signal level, at established distances, for instance, 80 to 100 km as one span. Moreover, the compensation node 104 will be inserted at intervals of several spans. On the compensation node 104, to solve the accumulation problem of the level deviation and dispersion slope deviation, an optical branching filter will be provided as an optical device to make level adjustment and dispersion compensation for each accumulation.
While at the terminal station B, the optical branching filter 105 will be provided. With this optical branching filter 105, the wavelength multiplexed optical signals sent through the optical transmission path fiber 102 will be separated on a wavelength-by-wavelength basis, and will be inputted to the applicable optical signal receiver 106 to regenerate signals.
FIG. 2 shows a configuration example of an optical multiplexer/branching filter as an optical device to be placed on the compensation node 104. The optical multiplexer/branching filter illustrated here has the component elements including the optical branching filter 1, optical attenuator 2 for level adjustment to be provided for every wavelength group, dispersion compensation fiber 3 and optical multiplexer 4.
On the optical branching filter 1, the input wavelength multiplexed optical signals will be once separated for each wavelength group. Then, with the optical attenuator 2, the optical signal level will be adjusted according to each wavelength group, and with the dispersion compensation fiber 3, the dispersion deviation will be adjusted, so as to allow the optical multiplexer 4 to synthesize signals again.
Here, the reason why optical signals will be separated or synthesized not on an individual wavelength-by-wavelength basis, but on a wavelength group-by-group basis is to reduce the quantity and size required for the optical attenuator 2 and dispersion compensator 3.
FIG. 3 illustrates wavelength groups. Individual wavelengths of two or more optical signals to be wavelength multiplexed are allocated on the wavelength axis. As illustrated, in order to separate and synthesize optical signals on a wavelength group-by-group basis, such as G1, G2, G3 and so on, the optical branching filter 1 is required to have a filter characteristic with a rectangular cutoff characteristic.
However, it is difficult to actually configure a filter having such a rectangular cutoff characteristic. Due to this difficulty, a guard band GB is provided to eliminate the need to place any optical signal at the grouped wavelength-to-wavelength. On an example illustrated in FIG. 3, the area for three wavelengths is considered a guard band GB area where any optical signal is not placed at the grouped wavelength-to-wavelength.
In addition, FIG. 4 shows the configuration of an optical multiplexer/branching filter, when add/drop of optical signals will be made with the compensation node 104. In FIG. 4, in order to make the optical signal add/drop function feasible, after separating optical signals on a wavelength group-by-group basis with the optical branching filter 1, add the branching filter 5 to further separate optical signals on a wavelength-by-wavelength basis.
It is also possible to add the multiplexers 6 and 7 and synthesize the inserted optical signals on a wavelength-by-wavelength basis, before synthesizing each wavelength group with the optical multiplexer 4.
Here, for the wave synthesizing/separation filter to be used on the optical branching filter 1 and optical multiplexer 4 for the optical multiplexer/branching filter on the compensation node 104, requirements are to ensure isolation with other ports and low loss characteristic.
Generally, the wave synthesizing/separation filter is actually configured with the multistage combination of the dielectric multilayer film filter 10 as illustrated in FIG. 5A. The dielectric multilayer film filter 10 utilizes the interference effect of the dielectric thin films alternately piled up on the circuit board. As for the band pass filter type as illustrated in FIG. 5A, against the incident port (1) for lights, this type has the transmission port (2) which outputs the specified wavelength light allowing it to pass through, and the reflection port (3) which cuts off and reflects wavelength lights other than the specified wavelength light.
Therefore, in FIG. 6 showing the transmission characteristic of the dielectric multilayer film filter, the components of the specified wavelength 1 of the optical signal to come in the incident port (1) will be outputted to the transmission port (2), and the components of other wavelengths than the specified wavelength 1 will be reflected and outputted from the reflection port (3).
FIG. 7 shows the conventional optical multiplexer/branching filter, as an optical device consisting of the multistage combination of the dielectric multilayer film filter 10 as illustrated in FIG. 5.
In FIG. 7, to provide functions as the compensation node 104, the optical attenuator 2 and dispersion compensation fiber 3 are provided between the optical branching filter 1 and optical multiplexer 4, and it is possible to obtain an optical device, the usage of which is not limited to the compensation node 104, by changing of the functional element to be provided between the optical branching filter 1 and optical multiplexer 4. The same applies to the following case.
In FIG. 7, the dielectric multilayer film filters 10-1 through 10-5, components of the optical branching filter 1, are sequentially connected in series, so that the reflected light (3) of the reflection port for the dielectric multilayer film filter comes in its incident port (1).
From the transmission port (3) for the dielectric multilayer film filters 10-1 through 10-5, the individual wavelength groups G1 through G5 will be allowed to pass through and will be outputted.
Moreover, the optical multiplexer 4 similarly consists of the dielectric multilayer film filters 10-6 through 10-10, and each dielectric multilayer film filter has three I/O ports, (1)′, (2)′ and (3)′, as illustrated in FIG. 5B, and its I/O characteristic is the reverse characteristic of the dielectric multilayer film filters 10-1 through 10-5 illustrated in FIG. 5A.
Therefore, the ports (1)′, (2)′ and (3)′ illustrated in FIG. 5B correspond to the ports (1), (2) and (3) illustrated in FIG. 5A, respectively.
Because of the reversible I/O characteristic, to the input port (2)′ on one side, the transmitted light to be outputted from the transmission port (3) applicable to the dielectric multilayer film filters 10-1 through 10-5 for the optical branching filter 1 will be inputted, and the transmitted light to be inputted will be directly outputted to the output port (3)′. In addition, the dielectric multilayer film filters 10-6 through 10-10 are sequentially connected in series, so that the output of the output port (3)′ for the dielectric multilayer film filter will be inputted to the input port (1)′ on the other side.
The light to be inputted to the input port (1)′ on the other side, will be reflected and outputted to the output port (3)′. Therefore, the optical signals of the wavelength groups G1 through G5 already separated by the optical branching filter 1 will be sequentially wavelength multiplexed with the dielectric multilayer film filters 10-6 through 10-10 for the optical multiplexer 4 and will be outputted.
Here, as the transmission rate is increasing, the flatness within the band of the multilayer film filter becomes important. If the flatness is not kept, waveform distortion may be caused, and the distortion can be accumulated by multistage transmission through the compensation node with the optical multiplexer/branching filter, thereby allowing the signal receiving characteristic to be deteriorated.
While, to the adjacent wavelength groups, the filter is required to ensure high isolation characteristic of 20 through 30 dB. If the amount of isolation is not sufficiently secured, the same signal may be synthesized by the output of the multiplexer 4, via different ports, and can be the components of coherent cross-talk. This could give interference noise to signals.
Therefore, in order to secure high isolation, the filter is required to secure a steep cutoff characteristic. However, as the characteristic of the dielectric multilayer film filter, a steep filter characteristic is hard to be compatible with a flat band characteristic. For this reason, it is necessary to provide a wide guard band GB where any signal is not placed at each wavelength group-to-group.
Here, define the method of grouping wavelengths as illustrated in FIG. 3, and represent the number of live channels within one group by m, and use n to represent the number of dead channels to be placed within the guard band GB at group-to-group, so that the relation is expressed by (m, n) as the band use efficiency, and finally find out the result, which found out to be (5, 3) in an example as shown in FIG. 3.
Accordingly, in case where the dielectric multilayer film filter is used, and the relation of the band use efficiency (m, n) is (6,3) or (5,3), the band use efficiency (=signal band width per one group/periodic wavelength intervals of grouped wavelength) will be about 60%, thereby highlighting the future problem to improve the band use efficiency compared with the system which does not need any dispersion compensation.
Moreover, as a pre-condition, the range in which the number of wavelengths that can be amplified will be restricted because of the band characteristic of the amplifier. Therefore, the more the number of dead channels to be placed within the guard band GB at the group-to-group increases, the fewer the number of effective wavelengths subject to amplification within the band characteristic of the amplifier could decrease.